Many fluid flow applications require that measurement and analysis of the fluid flow and other fluid characteristics be performed under adverse conditions. Consider, for example, the petroleum industry, where protection against overpressure of facilities such as vessels, piping, and machinery, etc., is provided by the use of safety valves at the various processing stations. The discharges from these stations are collected in headers and are directed from there into a main large discharge conduit. Typically, single headers collect discharges from a small number of safety valves, for example ten to twenty. A plurality of these single headers can be collected into a large collection header for an entire manufacturing unit. The gases are burned at an ignited flare or burner pit and from there are vented safely to the atmosphere.
At any particular time, many of the safety valves may be leaking. Usually, however, the leakage rate from any one valve is very small and of no great concern. At times, however, the valves may leak excessively, for example from operation at a pressure too slose to the safety valve setting, from mechanical damage during an overpressure incident, or from deterioration due to corrosion, erosion, fouling, or some other cause. Leakage from a safety valve can be costly due to the loss of a valuable product, degradation of the product being manufactured, or by creating problems from operation of the flare system. Furthermore, unless monitored, the leakage might not be corrected for an intolerably long period of time.
There have been many attempts to detect leakage in the collection headers. These previous methods have not proved successful. The malevolent atmosphere in the flare headers is one of the principal reasons. Thus, because the flare stack receives material from many sources, the process conditions are very hostile to instrumentation. Commonly, there can be fouling, corrosion, precipitation of solids, precipitation of high molecular weight polymers, and over the passage of time, various combinations of these conditions. Thus, for example, turbine meters have been employed but have proved inadequate due to clogging of their moving parts. Also, most ordinary engineering materials fail due to corrosion. Further, the impact resulting from sudden pressure surges, temperature transients, steam, long distances from sensors to electronics, vibration, etc., all combine, as noted above, to present an unfavorable environment which renders inoperable prior measurement systems. Furthermore, flow detection methods for leakage detection using differential pressure instruments are ruled out by the safety system back-pressure considerations which forbid pressure increases in the flare headers. In addition, the flow conditions within the header include both positive and negative directions of flow. The instrument systems noted above do not distinguish flow direction nor do they compensate for negative flow.
A further consideration in connection with measuring flare stack flow is the location of and limited access to existing pipes and the inability to take a particular pipe out of service merely to install the flowmeter. This means that it is important to be able to retrofit instruments such as flowmeters to an existing facility using, for example, an on-line hot tap procedure. Furthermore, the hot tap must be positioned exceptionally accurately so that subsequent fluid interrogation occurs along a predetermined path, for example at 45.degree. to the pipe axis, on a tilted diameter, or along a chord segment or in the axial direction at a prescribed distance from the pipe wall, etc., so that the sampled portion of the flow profile bears a calculable and/or reproducible relationship with the area averaged flow velocity.
In addition to the hostile environment presented by the flare stack headers, there is a further difficulty, even if instrumentation can be provided to withstand the hostile environment, that the gas flow characteristics within the header can change rapidly with time for a variety of reasons. Thus, the typical applications of ultrasonic flow measurement, such as that described in Lynnworth U.S. Pat. Nos. 4,103,551 and 3,575,050, are not directly applicable. In those references, it is generally assumed that the flow rate is relatively constant with time; that the fluid characteristics do not change violently; and that the composition and/or sound speed of the medium passing through the pipe is not only known, but constant, at least for intervals comparable to the measuring instrument response time. It is also frequently assumed that adjacent or neighboring fluid elements have substantially identical fluid characteristics so that crossed interrogation paths can be employed.
It is also often assumed that the received amplitude from measurements made upstream and downstream will be identical in amplitude. It has been shown however that the amplitude often differs depending upon whether the interrogation path is directed upstream or downstream. See for example Ingard, U. and Singhal, V. K., Journal of the Acoustical Society of America, Vol. 60, pp. 1213-1215, 1976.
It is therefore a primary object of the invention to reliably and accurately measure various characteristics of a flowing fluid particularly under adverse conditions such as the flow conditions in a collection header or in a main flare line. Other objects of the invention are the accurate measurement of fluid flow in spite of differences between the amplitude and/or shape of pulses transmitted downstream versus upstream, and despite density, pressure and turbulence variations, composition variations, and sound speed and flow rates which change quickly with time. Further objects of the invention are providing a measure of mass flow rate, fluid flow velocity, identification of the source of a leak, and installing flow measurement apparatus in a conduit with high accuracy and reliability of placement despite access restrictions.